This policy setting allows you to specify whether the Inbox application requires all messages to be encrypted. This policy ...

This policy setting allows you to specify whether the Inbox application requires all messages to be encrypted. This policy is applicable only if you are using Microsoft Exchange 2003 SP2 or Microsoft Exchange 2007 SP1. In order to use S/MIME, it must be enabled for use by OWA or EAS on the Exchange server.  Recipients must have a published public key (typically stored in Active Directory) accessible to the Exchange server in order to receive encrypted e-mail messages. A user who attempts to send an e-mail message to a recipient who does not have a published public key will receive an undeliverable message error.  If you enable this policy setting, then all messages must be encrypted.  If you disable or do not configure this policy setting, then messages do not have to be encrypted.